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Military Intelligence – this week in history September 27

The SIS used this analog of the Japanese Purple machine to decipher Japanese diplomatic messages in 1940.

Purple is much more than color for signals intelligence

Courtesy Photo

The SIS used this analog of the Japanese Purple machine to decipher Japanese diplomatic messages in 1940.

Sept. 25, 1940
By the spring of 1939, William Friedman’s teams of cryptologists had already achieved partial success with the Italian and German code-systems and the Japanese army systems. At the same time, the Navy had managed to break a number of the Japanese naval codes. But the biggest puzzle of all was the Japanese diplomatic cipher system, used by the Foreign Office in Tokyo for its most secret communications with its ambassadors serving abroad. This system was nick-named “Purple” – and it became the most legendary encipherment system in the history of cryptography prior to the computer age.

The first few Purple messages were intercepted in 1937. It took awhile for there to be enough traffic for the Signals Intelligence Service, or SIS, to even attempt to decipher them. The team expected the solution to follow the same pattern as the Enigma machine that the Germans had been using since the 1920s. They were quickly dismayed to find that the Japanese had implemented numerous ingenious adaptations and improvements to the Enigma, multiplying the difficulties of deciphering its code.

Ronald Clark, in his book “The Man Who Broke Purple,” explains the difficulty faced by Friedman and his expert team of SIS cryptologists:
“The first step in what seemed to be the impossible task of decipherment lay in collecting a certain minimum of traffic sent on one day – or, at least, sent with the same keys in operation, which was initially the same thing. The next step was to decide which permutation and combination of wiring could have been used to produce such a set of enciphered messages. If the decision was correct all messages sent on that particular day could be deciphered to produce intelligible plaintext. If not, not; and for month after month it was always not.”

It was slow, painstaking work. But gradually, the SIS team built up bits and pieces of deciphered traffic. In February 1939, General Mauborgne, the Chief Signal Officer, ordered Friedman to drop all of his other duties, and he and his staff concentrated solely on solving Purple. It became Friedman’s consuming passion, conducted in total secrecy, for an excruciating eighteen months.

In August 1940, the SIS had a breakthrough, which allowed them to build a mock-up of the Japanese encryption machine – called an analog. The Purple analog allowed the SIS analysts, to produce its first major, ungarbled, translated “solutions” of two PURPLE messages on Sept. 25, 1940. Ironically, this major success occurred just two days before Germany, Italy and Japan signed in Berlin the Tripartite Pact pledging all 3 of them to mutual support, an event which led to an unexpected, but useful, cascade of Purple messages. The Americans not only had the ability to read the diplomatic traffic, they had plenty of it to read.

Of course, there were plenty of issues that slowed down the process, such as the physical delay in passing encrypted intercepts from the field to the cryptographers in Washington, the small number of trained analysts handling a huge volume of traffic, and finding people who could translate accurately the deciphered messages. In spite of these obstacles, William Friedman had handed his government a tool which would change the course of history. As Clark writes, “By the autumn of 1940 there was available for the State Department, and for the Chiefs of Staff, a continuing flow of top secret information from Tokyo in which the Japanese leaders outlined their plans for the future and the strategy and tactics with which they were to be carried out.”

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